A satellite positioning system, such as the Global Positioning System ("GPS"), comprises a constellation of satellites that transmit signals that can be used by a wireless terminal to determine, in well-known fashion, the wireless terminal's position. Typically, the signals transmitted by each satellite convey three types of information: (1) satellite trajectory data, (2) system timing, and (3) ranging information. When a wireless terminal can acquire the signals from three or more satellites the wireless terminal can determine its position through triangulation, as is well-known in the art. FIG. 1 depicts a schematic diagram of a satellite positioning system in the prior art.
Although a conventional wireless terminal can determine its position with some degree of accuracy, fluctuations in the ionosphere and the atmosphere and jitter in the transmitted signals themselves prevent a conventional wireless terminal from determining its position with a high degree of accuracy. To mitigate the effects of these factors and thus improve the degree of accuracy with which a wireless terminal can ascertain its position, another satellite positioning system, typified by the Differential Global Positioning System ("DGPS"), was developed. FIG. 2 depicts a schematic diagram of a Differential Global Positioning System.
As is well-known in the prior art, DGPS comprises terrestrial reference receiver 205, whose position is static and exactly known through conventional survey techniques, in addition to satellite constellation 203 and wireless terminal 201. The theory underlying DGPS is that when wireless terminal 201 is in close proximity (e.g., within 50 miles) to terrestrial reference receiver 205, both wireless terminal 201 and terrestrial reference receiver 205 are expected to experience the same ionospheric and atmospheric fluctuations and signal jitter. Terrestrial reference receiver 205 uses the signals from satellite constellation 203 to estimate its position, and, using its known exact position, calculates the error between its estimated position and its known exact position. That error or "difference" is a vector that represents the inaccuracy of the estimated position from the ionospheric and atmospheric fluctuations and signal jitter. The difference vector is broadcast by terrestrial reference receiver 205 to wireless terminal 201 in real time. When wireless terminal 201 estimates is position through conventional means, it uses the difference vector received from terrestrial reference receiver 205 to subtract out the effects of the ionospheric and atmospheric fluctuations and signal jitter.
FIG. 3 depicts a schematic diagram of a Tidget.RTM. satellite positioning system in the prior art. The wireless receiver in a Tidget system does not compute the position of the wireless terminal. Instead, the wireless receiver in a Tidget system acts like a wireless repeater in that it receives the signals from the satellite constellation and then relays the unprocessed signals to a remote processing facility, which uses the signals to determine the position of the Tidget wireless terminal. An advantage of a Tidget system is that is reduces the cost of the wireless terminal by eliminating from the wireless terminal the expensive circuitry that would otherwise be needed to compute the position of the wireless terminal. When it is more advantageous that a remote facility know the location of the wireless terminal than that the wireless terminal know its own location, a Tidget system is advantageous in that it relays, in effect, the position of the wireless terminal to the remote facility.
FIG. 4 depicts a schematic diagram of a Tendler.RTM. satellite positioning system in the prior art. A wireless terminal constructed in accordance with this system comprises both the circuitry needed to determine its position from a satellite constellation and a wireless telephone transmitter to transmit the determined position to another party via a wireless telecommunications system.
Regardless of the advances made in satellite positioning systems, limitations still exist. Typically, the strength of the signals from the satellite constellation is too attenuated in buildings and other shadowed environments for a wireless terminal to receive. Furthermore, a wireless terminal can take several minutes to acquire the signals from the satellites it needs to determine its position.